When it comes to air purification, the best place to put a HEPA filter is in an open area with free air flow all around. This allows the filter to extract air from all directions and work with the best performance. The closer the unit is to your head, the shorter the distance the clean air will have to travel before reaching you. In the bedroom, this usually translates into placing the air purifier on a bedside table or bedside table near the bed.
Placing the purifier to the side of the room is ideal because of the way the particles diffuse into an area. As the air purifier cleans the air, the air around the purifier will be clean. The vacuum created by the fan will suck dirty air from across the room. In other areas of your home, it's best to place your HEPA filter near a window or door, as this allows for maximum airflow and helps remove dust, mold and other particles from your home.
Additionally, you should make sure that your filter is at least two feet away from any other objects on all sides, as this will ensure optimal airflow and air purification. If you have more than one device that filters, humidifies or dehumidifies the air or even an air conditioner, then you should consider which one to use. Using an air purifying pre-filter will prevent the main HEPA filter from becoming clogged with debris. Some air purifiers have powerful filters to eliminate odors and placing them near the odor source won't even smell the smell.
However, if your air purifier does not have a pre-filter, it is strongly recommended to add one to the air intake. Air purifiers don't really work well in high relative humidity environments (especially the activated carbon filter can quickly overload).